1. Field of the Invention
This invention relates to a nuclear well logging method of determining oil saturation of subsurface earth formations in the vicinity of a well borehole from gamma ray measurements obtained after irradiating the formations with high energy neutrons.
2. Description of the Prior Art
U.S. Pat. Nos. 3,748,474 and 3,562,523 related to methods for determining the oil saturation of an earth formation. Both methods were based on the measurement of gamma rays emitted as the result of the decay of captured thermal neutrons with different types of fluids being injected for different logs. For the techniques of these patents to work, the fluids to be injected were required to be either saline, because of the high thermal neutron capture cross section of chlorine, or contain added materials having elements with large thermal neutron capture cross sections. Further, the actual thermal neutron capture cross section for both the in situ formation fluid and the injected fluid had to be known.
In U.S. Pat. No. 3,780,302, an earth formation in the vicinity of the well borehole was irradiated in the in situ condition and measurements of the formation calcium, silicon, carbon and oxygen inelastic scatter gamma rays were obtained. With this technique, the inelastic scatter gamma rays from carbon, oxygen, silicon and calcium in the formation were detected and used as an indicator of the formation porosity. The carbon/oxygen ratio obtained could then be used as an indicator of the presence of hydrocarbons and a silicon/calcium ratio obtained could be used as an indicator of the formation lithology. By appropriately combining measurements of the carbon/oxygen ratio and a silicon/calcium ratio with estimates of the formation porosity, the oil saturation of the formation matrix surrounding the borehole could be computed and logged as a function of the borehole depth. However, knowledge of the borehole conditions (i.e. borehole diameter, casing thickness, type of borehole fluid) was required because of the adverse effect of variations in these conditions on the measurements of inelastic scatter gamma rays.